Unstable mental health may be the “fundamental, underlying cause” of a soldier’s shooting rampage at Fort Hood that left four people dead, though an argument with another service member likely preceded the attack, according to investigators.

An Iraq War veteran being treated for mental illness opened fire Wednesday on fellow service members at the Fort Hood military base, killing three people and wounding 16 before committing suicide at the same post where more than a dozen people were slain in a 2009 attack, authorities said.

An Iraq War veteran being treated for mental illness was the gunman who opened fire at Fort Hood, killing three people and wounding 16 others before committing suicide, in an attack on the same Texas military base where more than a dozen people were slain in 2009, authorities said.